The invention is especially concerned with determining the position and orientation of a helmet in a cockpit of an aircraft or helicopter. This is done to control aiming systems of weapons which are aimed in accordance with the direction in which the pilots head is pointing, and also because a head-up display is sometimes reflected via a combiner mounted on the helmet or via the visor itself, and the display may need to depend on the direction in which the head is pointing. Other uses may include the determination of position and orientation of a users head in a virtual reality system, or that of, say, a tool relative to a workpiece in a computer controlled machining centre.